What to pay a Bartender?

Friend of mine, her is sister is getting married in Vegas in August. They need to hire a bartender for the evening and have found a guy that I believes bartends in the Casino at City Center. He has offered to bartend for free and just put a tip jar out, but the couple would rather not have a tip jar and just pay him, what he would normally make in an evening.

Can someone provide me with a figure, that would be fair for an 8 hour shift? Whatevery they might make between salary and their tips?